1. Field of the Invention
The present invention relates to a method for controlling the operating condition of an internal combustion engine, and more specifically to a control method in which the operation of the engine is controlled by operating the fuel metering system on the basis of the vacuum level of the intake air passage, downstream of the throttle valve.
2. Description of Background Information
Among methods for controlling the operating condition of an internal combustion engine (referred to simply as an "engine" hereinafter), there is a type which controls the amount of the fuel to be injected to the engine.
In this type of control method, an absolute value of the vacuum level P.sub.BA in the intake air passage downstream of the throttle valve and the engine speed are detected, respectively by a pressure sensor and an engine speed sensor. Then a fundamental fuel injection duration Ti is determined in accordance with the output signals of these sensors. Further, an increment or decrement correction coefficient is multiplied to the fundamental fuel injection duration Ti, in accordance with other parameters such as the engine coolant temperature or a transitional change in the engine operation.
In the case of conventional control methods of this type, in determining the fundamental fuel injection duration Ti, a parameter corresponding to the absolute value P.sub.BA of the vacuum in the intake manifold is sampled in synchronization with a TDC signal which is indicative of the engine rotation, and the fundamental fuel injection duration Ti is determined in accordance with the thus sampled parameter values.
However, generally the absolute value P.sub.BA tends to pulsate during intake strokes of the engine, and in turn, the fundamental fuel injection duration Ti also varies with the pulsation of the absolute value P.sub.BA. This variation of the fundamental fuel injection duration Ti has resulted in unstable engine operation, especially in a low speed range of engine operation.